1. Field of the Invention
The present invention relates to copolymers useful for oil and gas production. The present invention particularly relates to copolymers useful for oil and gas production as acid gelling agents.
2. Background of the Art
The use of acid gels for the practice of acidizing oil and gas wells is well known. Acidizing, a procedure used to stimulate oil and gas production, is carried out by injecting a well servicing fluid, for example, a dilute solution of an acid such as hydrochloric acid, into a producing subterranean formation. The acid enters flow channels, such as cracks or fissures present in the formation, and enlarges the flow channels through which the oil or gas passes to the well bore, thereby increasing the production of the well. Matrix acidizing is carried out under relatively low pressure to enlarge or unplug existing channels by action of the acid against formations such as limestone. Fracture acidizing involves injection of the well servicing fluid under relatively high pressure to crack open existing fissures to create wider channels.
Regardless of the type of acidizing, it is desirable to incorporate a viscosifier into the fluid to enable injection of the fluid to smaller fissures as well as more open areas. The well servicing fluids including a viscosifier are often referred to as gels or fracture gels. In addition, with respect to fracture acidizing, a proppant such as sand is typically used. The viscosity of the gel functions to to keep the proppant suspended in the fluid during injection into the well being treated.
The polymers commonly used to gel acids for acidizing oil and gas wells include both natural and synthetic polymers. The natural polymers include the so called polysaccharide biopolymers. The synthetic polymers include those such as are taught in U.S. Pat. No. 5,975,206 to WOO, et al., hereinafter WOO. WOO discloses a crosslinked acid gel comprising an emulsion copolymer and an external activator and methods for acid fracturing subterranean formations using the crosslinked acid gels.
While other synthetic polymers, such as polyethers prepared with ethylene oxide or propylene oxide condensate backbones, can used for gelling acids, polymers having vinyl backbones are commonly used. For example, polymers prepared using the quaternary amine salts of dimethylaminoethyl methacrylate have been disclosed as suitable for gelling acids. Temperature Stable Acid Gelling Polymers, Norman, L. R.; Conway, M. W.; and Wilson, J. M., SPE 10260 presented during the 56th annual fall technical conference of the Society of Petroleum Engineers, San Antonio, Tex., Oct. 5-7, 1981.
The polymers described above are used to gel acids. Acid systems currently in use for acidizing oil and gas wells can be broadly classified into the following groups: mineral acids which includes hydrochloric acid, hydrofluoric acid and mixtures of hydrochloric-hydrofluoric acid; organic acids which include formic acid; and acetic acid; powdered solid acids such as sulfamic acid, and chloroacetic acid; mixed acid systems such as acetic-hydrochloric acid, formic-hydrochloric acid; formic-hydrofliuoric acid; retarded acid systems for example gelled acids, chemically retarded acids, and emulsified acids.
It would be desirable in the art of preparing and using acid gels in the field of oil and gas production to prepare such gels which have lower toxicity than conventional acid gels. If would also be desirable in the art if the gels had performance properties similar to conventional acid gels.